A p-styrenesulfonic acid (salt) represented by sodium p-styrenesulfonate is a functional monomer having a radical polymerizable vinyl group, a hydrophobic benzene ring having π electrons, and a sulfonic acid (salt) group that is a strong electrolyte, and has been valued in various industrial fields. For example, it has been used as a reactive emulsifier in order to improve stability of an emulsion and water resistance or cationic dye dyeability of a (emulsion) polymer. Further, a polymer or a copolymer of sodium p-styrenesulfonate has been used as a dispersant for producing various aqueous dispersions of pigments, antioxidants, various polymers (tackifier resins, chloroprene rubber, polyacrylic acid esters, polyesters, styrene-butadiene copolymers, polyvinyl chloride, poly-acrylonitrile, polysilicones, conductive polymers and the like), nanocarbon materials, silica particles for hot forging release agents or abrasives, battery electrode materials (carbon, lithium iron phosphate, lithium manganese phosphate and the like), photographic silver halides and the like. Furthermore, the polymer of sodium p-styrenesulfonate has been utilized as synthetic starch for clothing finishing such as an ironing agent, a hair care product, an antistatic agent, a resist acid generator, a water treatment agent, an allergen scavenger, an ion-exchange resin, a plating solution additive, a detergent for production of semiconductors and hard disks, an additive to fluids for shale oil drilling and a flame retardant for resins.
In the above-mentioned application fields, various improvements have been required for sodium p-styrenesulfonate and the (co)polymer thereof. Among these, an improvement need common to many uses is hue. The hue of sodium p-styrenesulfonate and the (co)polymer thereof is an important factor directly exerting an effect on the commercial value, particularly in uses of an adhesive, a polymer emulsion for pigments, synthetic starch for clothing finishing such as an ironing agent, an antistatic agent, a frame retardant for transparent resins, a photographic silver halide emulsion and the like. That is to say, conventional sodium p-styrenesulfonate industrially available and the (co)polymer thereof exhibit a pale yellow to pale yellowish brown color, and it has been strongly desired to make them light-colored or colorless.
About the hue of sodium p-styrenesulfonate, an influence of impurities is suggested (for example, Patent Document 1). However, about the most important impurities, metal halides are only mentioned, and no other specific hue is mentioned in any way. Further, about the hue of the (co)polymer of sodium p-styrenesulfonate, an influence of a polymerization initiator is suggested (for example, Patent Document 2). However, iron and other impurities contained in sodium p-styrenesulfonate as a raw material and an influence thereof on the hue are not mentioned in any way.
Further, chloroprene rubber-based adhesives have long been used as all-purpose adhesives. Methods for producing chloroprene rubber are publicly known. For example, chloroprene or chloroprene and a radical polymerizable monomer copolymerizable therewith are emulsified in water using an alkali metal salt of disproportionated rosin acid (acting as an emulsifier) and an alkali metal salt of a naphthalenesulfonic acid formalin condensate (acting as a dispersant), and a radical polymerization initiator is added thereto to perform emulsion polymerization. The unreacted monomer(s) in a chloroprene rubber emulsion obtained is removed by a steam distillation process. Finally, chloroprene rubber is taken out of the emulsion by a process such as freezing coagulation, and washed with water and dried, thereby being able to produce solid chloroprene rubber (for example, Non-Patent Document 1). The chloroprene rubber-based adhesive is produced by dissolving the above-mentioned chloroprene rubber and compounding agents such as a tackifier resin, a metal oxide and a crosslinking agent in an organic solvent such as toluene, methylcyclohexane, n-hexane, methyl ethyl ketone or an acetate ester. When the chloroprene rubber-based adhesives are used for production of footwear or sporting goods, an excellent hue (to be as close to colorless as possible) is required.
Accordingly, as a method for improving the hue of the adhesives, there has been proposed a method using as a dispersant an alkali metal salt of a styrenesulfonic acid (co)polymer that is hardly discolored, instead of the above-mentioned alkali metal salt of the naphthalenesulfonic acid formalin condensate (for example, Patent Document 3). Surely, the hue is improved. However, it is not necessarily satisfactory, and a further improvement has been required. Further, the hue of the poly(sodium p-styrenesulfonate) (co)polymer used is not mentioned in any way.
Furthermore, as synthetic starch used for a clothing finishing agent such as an ironing agent, there has been known poly(Sodium p-styrenesulfonate) (Patent Document 4). The hue is extremely important in this use, and it has been required to make the synthetic starch more colorless. However, the hue of poly(sodium p-styrenesulfonate) is not mentioned in any way.
On the other hand, it has previously been known that sodium p-styrenesulfonate can be produced by the reaction of p-β-bromoethylbenzenesulfonic acid and sodium hydroxide (see Patent Documents 5 and 6).
Patent Document 5 describes a method for producing a hemihydrate of sodium p-styrenesulfonate by charging a reactor with an aqueous sodium hydroxide solution containing a slight amount of sodium nitrite as a polymerization inhibitor under a nitrogen atmosphere, performing the reaction at 90° C. while adding dropwise p-β-bromoethylbenzenesulfonic acid thereto to obtain crystals of sodium p-styrenesulfonate, and thereafter performing cooling, centrifugal filtration and forced fluidization. It is only described that the particle size of sodium p-styrenesulfonate is usually from several micrometers to several millimeters, and no observed value is described. It is assumed to have a particle size equivalent to that of one on the market at present, that is to say, a median diameter of about 20 μm. Further, the relationship among the particle size, water content, solubility and fluidity is not mentioned in any way.
Patent Document 6 describes a method for producing a hemihydrate of sodium p-styrenesulfonate by performing the reaction at 90° C. while concurrently feeding an aqueous sodium hydroxide solution containing a small amount of sodium nitrite as a polymerization inhibitor and p-bromoethylbenzenesulfonic acid to a reactor, continuously taking out a sodium p-styrenesulfonate crystal slurry formed, and performing centrifugal filtration and forced fluidization. It is described that the shape of sodium p-styrenesulfonate is scale-like and that the observed value of particle size is from 160 μm to 760 μm. Further, similarly to Patent Document 5, the relationship among the particle size, water content, solubility and fluidity is not mentioned in any way.
However, conventional sodium p-styrenesulfonate having a median diameter of less than 25 μm has had a problem with handling properties, such as the occurrence of clogging in a charging hopper at the time when used in large amounts in a plant. That is to say, it has been lacking in fluidity as a powder. On the other hand, when the median diameter of sodium p-styrenesulfonate exceeds 150 μm, the rate of dissolution in water has decreased at the time when used in a plant to cause a problem such as clogging of a strainer, although satisfactory in fluidity. That is to say, sodium p-styrenesulfonate improved in fluidity without impairing the solubility of sodium p-styrenesulfonate has been required.
Further, in the above-mentioned detergents for production of semiconductors and hard disks, antistatic agents of films for electronic materials, dispersants for conductive polymers, photographic silver halide emulsifiers and the like, particularly in electronic material uses, it has recently been required to decrease impurities such as sodium bromide contained in p-styrenesulfonic acid (salts) and p-styrenesulfonic acid (salt) polymers, nuclear bromides of p-β-bromoethylbenzenesulfonic acid and p-styrenesulfonic acid, and excess metals and bromine components. It has been known that the impurities such as sodium bromide can be decreased by washing or recrystallization with an aqueous solvent (for example, Patent Document 7). Although the purity is described in Patent Document 7, the contents of alkali metal halides are not clear, and there is no description for the other impurities at all. Further, an effect of the particle size of sodium p-styrenesulfonate before purification on the purification efficiency, namely the purity after purification, is not mentioned in any way.